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The template to declare asset catalog (imageset, appicon, launchimage)
copy the resources into a .xcassets directory in $target_gen_dir, thus
there is no need to put them in a .xcassets directory in the checkout.

Standardize the asset catalog declarations so that the asset catalogs
are in a resources/ sub-directory, the corresponding target is in the
resources/BUILD.gn file.

Bug: none
Change-Id: I56a751add67f7d77e95dcb0b213dfd756f129310
Reviewed-on: https://chromium-review.googlesource.com/672704Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502863}